Anthropogenic greenhouse gas emissions are the main concern for global warming and must be urgently addressed. The use of Ca-based absorbents is among the options in CO2 capture technology, and it can be combined with waste H2S to create a better capture capacity. Here, we use decomposed phosphogypsum (PG) as a Ca-based absorbent to solve the problem of PG resource utilization, along with CO2 capture. Furthermore, the H2S exhaust gas generated during the carbonation process is reused to further promote capture capacity of the absorbent. Different liquid-solid ratios (L/S) and thermal parameters are investigated to determine the optimal conditions. A relatively low L/S value (6:1) can better capture CO2. Meanwhile, the CO2 capture capacity and H2S concentration were measured to explore the mechanisms of the carbonation process. X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyzed the chemical components of both reactants and resultants. The morphology of PG and CaCO3 are rhombus and calcite shapes, respectively. Most importantly, sulfur looping (S-looping) routes and the reaction process mechanism of this decomposition/carbonation cyclic system are determined and outlined, indicating that S-looping occurs throughout the entire experiment.